1. Field of the Invention
The present invention relates to a logic circuit, and more particularly to a logic circuit which is provided with an active pull-down circuit for the purpose of reducing the transition time from a high level to a low level of the output of an emitter-follower circuit.
2. Description of the Prior Art
In order to drive a load an emitter-follower circuit is used in general as the output stage of an ECL circuit or the like. In such a case, the emitter-follower circuit is biased using a static current source. When a driving signal is switched from a low level to a high level, the emitter-follower circuit pulls up the output to the high level. The switching speed at this time is proportional to the current that flows in the transistor. On the contrary, when the logic level is switched from the high level to the low level, the pull-down to the low level is accomplished through the drawing of the relevant current by the static current source.
In such an output stage, the probability for the appearance of a malfunction is increased because of the difference in the switching times at the rise and at the fall of the output. For this reason, the transition time from the high level to the low level of the output has generally been reduced by providing the emitter-follower circuit with an active pull-down circuit.
In a logic circuit having an emitter-follower circuit provided with the conventional pull-down circuit, the transition time in the fall of the output will be smaller for greater sinking power of the active pull-down circuit. However, if the load capacitance connected to the output terminal is too small for the sinking power of the pull-down circuit in this case, the undershoot at the time of transition of the output level becomes large. As a result, there arise such problems as causing malfunctions in the logic circuit in the next stage and a prolongation of the transition time from the low level to the high level of the output as well as other problem such as a wasteful flow of current via the active pull-down circuit even after the exhaustive discharge of the electric charge on the load capacitor that is connected to the output terminal owing to the large sinking power of the pull-down circuit, resulting in an increase in the power consumption of the circuit as a whole.